Microfluidic systems, such as lab-on-chip systems and microfluidic devices for biomedical detection systems, have been recently proposed which combine silicon chip packaging and flexible electronics technology. Such systems can be used for example as wearable sensor systems, such as sweat sensor patches and wound patches for fluid analysis and monitoring. Recent works describe such systems where a microfluidic micropump device is used to provide a microfluidic transportation (i.e. a continuous fixed fluid flow) for example from the fluid reservoir to the sensor system. Recent works describe micropump devices using a combination of evaporation and capillarity that run passively for a long time and that can be realized at reduced production cost and with simple processes and materials.
C. Nie et. al. describe in “A microfluidic device based on an evaporation-driven micropump” (Biomedical Microdevices (2015), vol. 17, issue 2, April 2015) an evaporation-driven micropump, which can be used to provide a continuous flow through a microfluidic channel. The micropump device is characterized with a fluid inlet and a fluid outlet. When a fluid is provided through the micropump device, it flows through the channel and evaporates through the fluid outlet. The fluid outlet is covered with a micro-porous membrane, which is designed such that the micropump device pumps the fluid through the channel by evaporating through the outlet with a fixed flow rate. To achieve a fixed flow rate the micro-porous membrane covering the outlet is designed to provide an evaporation rate exactly equal to the desired flow rate. In other words, the evaporation rate through the micro-porous membranes defines the flow rate with which the fluid is pumped through the micropump device.
However, there is a need for a microfluidic device which is able to provide information about the fluid flow rate. This is especially important where for sweat sensing applications or for other applications for which the readout performance is affected by the fluid flow rate.